1. Field of the Invention
The present invention relates to techniques for communicating between integrated circuit chips. More specifically, the present invention relates to a method and an apparatus for communicating between integrated circuit chips that are arranged face-to-face through optical signaling.
2. Related Art
Advances in semiconductor technology presently make it possible to integrate large-scale systems, including tens of millions of transistors, into a single semiconductor chip. Integrating such large-scale systems onto a single semiconductor chip increases the speed at which such systems can operate, because signals between system components do not have to cross chip boundaries, and are not subject to lengthy chip-to-chip propagation delays. Moreover, integrating large-scale systems onto a single semiconductor chip significantly reduces production costs, because fewer semiconductor chips are required to perform a given computational task.
Unfortunately, these advances in semiconductor technology have not been matched by corresponding advances in inter-chip communication technology. Semiconductor chips are typically integrated onto a printed circuit board that contains multiple layers of signal lines for inter-chip communication. However, signal lines on a semiconductor chip are about 100 times more densely packed than signal lines on a printed circuit board. Consequently, only a tiny fraction of the signal lines on a semiconductor chip can be routed across the printed circuit board to other chips. This problem creates a bottleneck that continues to grow as semiconductor integration densities continue to increase.
Researchers have begun to investigate alternative techniques for communicating between semiconductor chips. One promising technique involves integrating arrays of capacitive transmitter plates and receiver plates onto semiconductor chips to facilitate inter-chip communication. If a first chip is situated face-to-face with a second chip so that transmitter plates on the first chip are capacitively coupled with receiver plates on the second chip, it becomes possible to transmit signals directly from the first chip to the second chip without having to route the signal through intervening signal lines within a printed circuit board. It is also possible to communicate in a similar manner through inductive coupling by using wire loops to couple magnetic fields between chips.
Unfortunately, both capacitive and inductive coupling mechanisms decrease in strength by approximately the inverse of the distance between the chips. The decreased strength of the received signal reduces robustness of the communication mechanism and increases the complexity, power, and area of the receiver and transmitter circuits. Additionally, capacitive and inductive coupling mechanisms suffer from cross-talk, or destructive coupling, from adjacent channels that increases with the distance between chips. It may consequently be difficult to assemble chips into a system with sufficient precision to allow capacitive and inductive coupling mechanisms to operate effectively.
Hence, what is needed is a method and an apparatus for communicating between semiconductor chips without the above-described problems.